Continuous flow systems are an attractive solution for processing large volume blood samples within the framework of microfluidic devices. However, existing continuous flow separation mechanisms are not well suited for handling high hematocrit-level samples. Hematocrit refers the percentage of whole blood that is made up of red blood cells. Non-batch microfluidic devices generally require hematocrit levels ranging from 3% to 25% with the raw blood sample requiring dilution with buffer, and therefore necessitating longer device runs and perhaps additional integrated enrichment or capturing processes.
Centrifugal or compact disc (CD)-based microfluidics offer a simple approach to blood sample preparation as they allow for the multiplexing, automation and miniaturization of the classical blood separation technique based on sedimentation. By exploiting density and size differences between the various blood components, one sediments the denser cellular components of blood and is left with a cell-free plasma sample. The small-volume separation of plasma from both raw and diluted blood samples using centrifugal microfluidic systems has been demonstrated using a CD-based platform. For example, Schembri et al., have demonstrated a multiplexed centrifugal microfluidic device capable of processing a 90 μL whole blood sample by separating and then diluting the plasma into 12 separate testing chambers. See Schembri C. et al., Centrifugation and capillarity integrated into a multiple analyte whole blood analyser. J. Automat. Chem. 17, 99-104 (1995). Additionally, Haeberle et al. developed a device to extract 2 μL of plasma from 5 μL of a whole blood sample. Haeberle S. et al., Centrifugal extraction of plasma from whole blood on a rotating disk. Lab Chip 6(6), 776-781 (2006). They demonstrated a separation time of about 20 seconds at a moderate spinning frequency of 40 Hz (2400 RPM). The same group also integrated measurement of the concentration of hemoglobin (Hb) or hematocrit levels in human whole blood samples with plasma separation.